Conveyor belts are used in literally hundreds of different applications ranging from the movement of large pieces of equipment between one production operation and another, to the transportation of persons from point A to point B, to many warehousing operations for placing inventory into stock and removing the inventory from stock. The continuous movement of conveyor belts over and around rollers, along with the repetitious start-ups and slow downs, the placing of loads thereon, and the removal of the loads therefrom, all cause wear, tear and fatigue to the belt. Fatigue and breaking of the belt is most likely to occur in the area where the ends of the belt are joined or spliced together. With the frequent changes of speed and the loads that are being carried by the belt, the belt is constantly being subjected to tension, often resulting in stretching or elongation of the belt. When a conveyor belt becomes elongated to the point where it is no longer able to be used without excessive slipping or removal from the beltway, the belt must be replaced. An alternate option is to stop the production line, remove a section of the belt and rejoin the ends of the belt. The time during which a conveyer line is shut-down while the belt is being repaired or replaced is non-productive and can contribute to increased overhead costs.
Conveyor belts are made from a wide variety of materials to meet a myriad of differing needs. The type of belt to which the present invention is applicable is the kind utilizing a polyamide core covered with one or more layers of a synthetic rubber or polymer such as polychloroprene, polyurethane, etc.
The polyamide core is preferably selected from the group comprising condensation products of adipic acid and hexamethylenediamine, and polymerized caprolactum.
Conveyer belts are fabricated according to techniques that are well known and documented in the art; thus, these techniques do not comprise any portion of the present invention. Typically, the core or carcass of the belt is comprised of a polymer in the form of elongated fibers or strands or as a solid sheet having an orientation in the longitudinal direction of the belt to provide high strength and elongation resistance. Nylon, a polyamide, in the form of fibers, strands, fabrics or a solid flexible sheet is commonly used as a core material because of its high breaking strength and low module of elasticity.
The top surface of the core is covered with an elastomeric coating having a co-efficient of friction which is suitable for the intended end use of the belt. The bottom surface of the core typically is covered with a layer of an elastomeric compound having the flexibility and wearability to repeatedly pass over around and through rollers without premature failure. Each surface of the core is covered simultaneously or sequentially by well known techniques such as passing the cover layer and core through pinch rolls to laminate the two together, using heat and/or adhesives to produce a strong bond. Among the compounds found to be useful for the surface coating on conveyer belts are natural rubber, butyl rubber, neoprene, styrene-butadiene rubber, polyvinyl chloride, ethylene-propylene rubber, polyesters, and polyurethane.
In the past, a variety of methods has been used to join or butt together the two ends of the belt. Some of these methods have employed mechanical means such as staples, flexible links, and tongue and groove joints, while others are based on the use of suitable chemical bonding agents such as hot melt adhesives to join together the ends of the belt.
Among the hot melt adhesives used for this purpose are thermoplastic polymers such as polyamides, polyolefins or polyesters modified with waxes, processing aids and other additives to change or control the flow characteristics of the adhesive. In use, the adhesives are heated to temperature at which they become soft and/or flowable. When cooled, they solidify to bond together the ends of the belt.
One problem arising from the use of belts comprising two or three different materials is finding a common adhesive that can be used to bond together the ends of the polyamide core material and to bond the surface coatings to themselves. Often, it is found that two separate cements must be used, one used to bind the core and another to cement the surface coatings. Another drawback with the use of these cements is that many are solvent based cements rather than water based. This contributes to a greater need for adequate ventilation during the splicing process, particularly if heat is used. If a flammable solvent is used in the cement or for cleanups, extra precaution is needed to prevent accidental flash fires.
In addition, it has been found that there is a tendency of the edges of the belt to curl, when the use of two separate cements is required. This curling of the edges may result in premature failure of the splice. Still another problem is the tendency of the belt, in the area of the splice, to be less flexible than the remainder of the belt, resulting in undue stress and strain on the belt particularly in the area of the splice. Yet another problem is the tendency of the belt, in the area of the splice, to become elongated, thereby resulting in early failure or slippage of the belt particularly around the drive roller. Coupled with these drawbacks is the added time required to repair the belts when using two types of adhesive cements.